332 M. K. jjurnstcin on tlte Injiuence of 



papor have slio^vn that exactly tlie same behaviour is exhibited 

 by the five substances which I have examined, viz. silver, 

 platinum, copper, gold, and aluminium ; and therefore it is 

 highly probable that the analogy holds for all other metals. 

 All the metallic lamina? employed were laid upon rectangular 

 glass plates, the two halves of which were coated with different 

 metals. The dimensions of these plates were 100 by 80 milli- 

 metres, except the plates coated with platinum, the edges of 

 which were 45 and 70 millimetres in length. The silver 

 was deposited by Martin's method ; the platinam was burnt 

 in on plate glass ; the copper, the gold, and the aluminium 

 were in thin leaves, such as are used by gilders and may be 

 purchased in the shops. These leaves can be floated on 

 water and then lifted out by means of the glass plate which 

 is to be coated ; and then, if the drops of water still clinging 

 to the plate be removed by a gentle heat, the metallic lamina 

 adheres firmly to the glass. In the middle of the plate, where 

 the two metals met, they overlapped to a distance of from 15 

 to 20 millimetres. Of the different laminse, the silver appeared 

 blue, the platinum grey, and the gold green by transmitted 

 light, while the copper and the aluminium were not trans- 

 parent. A chemical analysis showed that the copper con- 

 tained small quantities of tin and traces of iron ; the gold 

 was, as far as could be ascertained, pure ; and the aluminium 

 contained traces of iron. For the purpose of making contact 

 with the metallic laminae, small strips of tinfoil were pasted 

 on to the narrow ends of the glass plates. The layers of 

 silver and platinum were connected with the tinfoil by little 

 strips of aluminium, while the leaves of copper, gold, and 

 aluminium rested upon the edges of the strips of tinfoil and 

 thus completed the electrical contact. Copper wires were 

 soldered to both the strips of tinfoil on each plate. Lastly, 

 strips of black paper were pasted over the junctions of the 

 tinfoil and the metallic lamina which was to be examined, 

 the blackened face being towards the metal, so as to prevent 

 any rays reaching these junctions ; and then the plates were 

 ready for an experiment. 



The observations were made with a galvanometer having an 

 astatic nedele, a mirror, and a scale ; and the galvanometer 

 was connected with the plate under examination by a com- 

 mutator, through which the circuit was completed. The pre- 

 sence of any electromotive force could be unmistakably 

 proved by turning the commutator; and the readings corre- 

 sponding to reversed positions of the commutator differed 

 ahvays by twice the amount of the deflection to be observed. 

 The position of equilibrium of the galvanometer-needle was 



